1. Field of the Invention
The invention relates in general to color cathode ray tubes and more particularly to a color cathode ray tube of the current dependent type.
2. Description of the Prior Art
Color cathode ray tubes used in general color television receivers utilize an electron beam which passes through a shadow mask, an aperture grill or similar structure which is located adjacent a phosphor screen so that the electron beams corresponding to the respective colors impinge on the phosphor dots or stripes of the respective colors formed on the color phosphor screen so as to produce a color image.
Color cathode ray tubes of the so-called current dependent type are known which have no electron beam landing position determining means. For these tubes, the color phosphor screen is formed by mixing and coating phosphors of at least two colors which have luminance characteristics versus current density which are different from each other. Thus, when the electron beam current density from the common electron beam source changes or in practice the cathode current value is varied, the light emission of a predetermined hue is obtained.
Since current dependent type color cathode ray tubes include no electron beam aligning and blanking means, the cathode ray tube can be light weight and the manufacturing and assembling processes can be very simple. There is also a further advantage in that the resolution can be improved and color misregistration caused by the relative position displacement between the phosphor screen and the electron beam landing position determining means are avoided, since there are no electron beam landing position determining means in such tubes.
The characteristics of the electron gun in the practical color cathode ray tube are such that correspondence between the cathode current and the current density is not linear so sufficiently high color purity cannot be obtained with such prior art cathode ray tubes.
The copending patent application entitled "Current Sensitive Color Cathode Ray Tube" filed June 7, 1982 Ser. No. 385,831 assigned to the assignee of the present invention discloses a current dependent color type cathode ray tube in which the color phosphor screen is formed of phosphor whose luminance or brightness characteristics versus current density is a so-called sublinear characteristic as illustrated by curve 1 in the graph of FIG. 1 and which emits red light. The phosphor having a so-called super-linear characteristic shown by curve 2 in the graph to FIG. 1 emits green light and the above two different phosphors are mixed together and laminated one on the other. The current density of the electron beam which strikes the color phosphor screen and which is varied by the cathode current is switchable to selected values shown by A, B and C in the graph of FIG. 1. When the current density is selected to have a value A, the light emission of the red color determined by the characteristic 1 at a point a is dominantly made. When the current density is selected to have the value B, the light emission determined by the intersection of the characteristics 1 and 2 at a point b will occur which is the light emission of yellow as an intermediate color between red and green occurs. When the current density is selected to have the value C, although the light emission determined by the characteristic 2 at point c is made dominantly the light emission of yellowish green caused by the light emission by the characteristic 1 is obtained. Thus, when the beam current density is selectively changed in response to a color signal, a color image can be reproduced on the color phosphor screen.
The current density is changed by changing the cathode current. However, in practice, when the cathode current Ik is changed, the spot diameter of the beam formed on the phosphor screen is also changed. The relationship between the cathode current Ik and the spot diameter of the beam is illustrated by curve 3 in the graph of FIG. 2 in which as the cathode current Ik increases, the spot diameter of the beam also increases. This relationship is not linear so that the relationship between the cathode current Ik and the current density at the beam spot will not be linear as is illustrated in curve 4 in the graph of FIG. 3. Thus, if the value of the cathode current Ik is varied within the range from a value D to a value E illustrated in FIG. 3, the current density is changed in a relatively small range from a value F to a value G. Thus, in this case, the cathode current Ik is selected to have a value of E and the current density C shown in the graph of FIG. 1 will be obtained. If the cathode current is selected to be the lower limit value D, the current density cannot be made small enough to satisfactorily operate. The current density cannot take a value so as to produce the red light emission shown in the graph of FIG. 1, and hence the color purity particularly the red color purity for this example, is lowered.